The present invention relate generally to vehicle navigation systems and more particularly to a vehicle navigation system with improved startup performance.
Known vehicle navigation systems generally include a plurality of motion and position determining devices, such as a GPS receiver, accelerometers, gyros, wheel speed sensors, etc. The navigation system further includes a CPU receiving data from all of the sensors and having a hard drive containing a database of roads which may be traveled by the vehicle. As is generally known, the navigation system determines the position of the vehicle relative to the database of roads and displays the current position of the vehicle on a display. Further, the driver can select a destination relative to the database of roads. The navigation system then displays turn-by-turn instructions to the driver to guide the driver to the selected destination.
In order to conserve battery power, navigation systems generally enter a "low power" mode when the vehicle engine is not running. In low power mode, the hard drive is not spinning, the CPU is not booted, the sensors do not consume power and the GPS receiver does not receive GPS signals. To determine whether the engine is running, the current systems are connected to the "ON" position of the vehicle ignition switch. When the ignition switch is turned to the ON position, the navigation system switches from low power mode to full power mode. At that time, the hard drive in the navigation system starts spinning, the CPU is booted, the motion sensors begin to warmup and stabilize, and the GPS receiver receives GPS signals and begins calculating a GPS position solution. This "warmup" period by the navigation system components requires time during which the driver of the vehicle may be taking other actions that the navigation system needs to monitor in order to know the current position of the vehicle. It would be difficult and undesirable to force the vehicle to remain still while the navigation system is in the warmup period. In cold weather, this warmup period could be 30 seconds or more.
Additionally, startup performance of the navigation system is closely related to the GPS receiver's "time to first fix." To minimize this time, it is important for the GPS receiver to have fresh satellite ephemeris data. This data, which is used to determine the precise location of the GPS satellites, typically needs to be refreshed at least every four hours. If the vehicle is parked for more than four hours, the GPS receiver must first obtain fresh ephemeris data before calculating an accurate position solution. This further delays the startup performance of the vehicle navigation system.